Medical x-ray imaging systems up until recently were typically based on either film or phosphor. Newer systems based on large area flat panel technology or storage phosphors enabled production of digital x-ray images. These newer systems, while providing many advantages have considerably increased initial cost compared to older film based systems.
Active Matrix Flat Panel Imagers are based on phosphor or photoconductor detectors with direct electronic readout of individual pixels. They can provide images very quickly and are therefore very useful for demanding applications, like fluoroscopy. However, Active Matrix Flat Panel Imagers are extraordinarily expensive compared to the systems they are replacing and are capable of more than is needed for some applications, like radiography. Different approaches have been considered to make a low-cost x-ray digital imaging device for general radiography.
For example, photostimulable phosphors, or storage phosphors, are currently used in Computed Radiography systems. When exposed to x-rays these phosphors capture an image, which is later released by shining a specific colour of light onto the phosphor. The readout process is separate from the exposure and makes use of a large readout device. In order to modify a Computed Radiography system for immediate readout, the reader must be made very compact with a line scanning system as opposed to current spot scanning technology, which will increase the cost of the system significantly. These systems have very poor image quality, requiring approximately four times as much radiation to achieve the same image quality as Active Matrix Flat Panel Imagers. The reason for this drop in quality is the combination of the relatively poor x-ray absorption of Computed Radiography screens and the signal losses in the reader.
Other approaches exist for digital x-ray imaging but thus far none provides the quality of Active Matrix Flat Panel Imagers at a significant cost reduction. One such method is scanned projection x-ray, where a slot reader receives a matching x-ray beam. However, generation of narrow x-ray beams is difficult and mounts significant costs. Another method is an optically demagnified x-ray screen coupled to a camera system. This system is ultimately limited by the resolution of the camera system, and the signal loss between the screen and the camera. Another approach is to directly readout the charge on a selenium plate. However, this results in large amounts of noise.
X-ray imaging systems based on a photoconductive detector layer and an electro-optic light modulator have also been attempted. Known prior art includes the apparatus described in references 1 to 3, U.S. Pat. Nos. 6,052,432 and 5,847,499, and CA patent 2,228,325; The suggested approach thus far suffers from problems characteristic for optically-demagnified screen-camera systems. Although the image created is of high quality, and in some cases, can be acquired by the camera without significant loss of signal, the described configurations are only practical for small-area detectors. The suggested implementations do not allow the imaging system to be scaled up to the size required for radiography without sacrificing pixel density, image quality or the overall cost.
Furthermore, the decay of the image in the electro-optical light modulator present in the devices disclosed in U.S. Pat. Nos. 6,052,432 and 5,847,499, and CA patent 2,228,325 and the acquisition of multiple images to improve the image quality or the dynamic range has been proven to be impractical thus far. Work has also been done on Polymer Dispersed Liquid Crystal based systems (see references 4 and 5). Those devices also suffer from the problems described above along with rapid image degradation due to the high ionic content of Polymer-Dispersed Liquid Crystals.
It is therefore an object of the present invention to provide a novel development of a digital radiographic system based on a photoconductive detector and an electro-optic light modulator designed to store the image in the electro-optic light modulator and a method of x-ray imaging associated with it.